Revenue Recognition - Disaggregation of Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 501,519	$ 467,777	$ 1,484,886	$ 1,304,355
Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients	206,646	188,772	615,859	523,661
Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients	223,173	210,919	667,711	580,836
Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients	71,700	68,086	201,316	199,858
U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	279,314	271,609	820,116	715,986
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	76,393	64,910	232,274	194,778
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	27,623	23,046	74,598	79,060
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	383,330	359,565	1,126,988	989,824
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 118,189	$ 108,212	$ 357,898	$ 314,531
Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	100.00%	100.00%	100.00%	100.00%
Customer Concentration Risk | Revenue from Contract with Customer | Time-and-Materials
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	40.00%	41.00%	40.00%
Customer Concentration Risk | Revenue from Contract with Customer | Fixed-Price
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	45.00%	45.00%	45.00%	45.00%
Customer Concentration Risk | Revenue from Contract with Customer | Cost-Based
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	14.00%	15.00%	14.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	56.00%	58.00%	55.00%	55.00%
Customer Concentration Risk | Revenue from Contract with Customer | U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	15.00%	14.00%	16.00%	15.00%
Customer Concentration Risk | Revenue from Contract with Customer | International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	5.00%	5.00%	5.00%	6.00%
Customer Concentration Risk | Revenue from Contract with Customer | Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	76.00%	77.00%	76.00%	76.00%
Customer Concentration Risk | Revenue from Contract with Customer | Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	24.00%	23.00%	24.00%	24.00%
Energy, Environment, Infrastructure, and Disaster Recovery
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 204,802	$ 179,002	$ 596,089	$ 522,698
Energy, Environment, Infrastructure, and Disaster Recovery | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	41.00%	38.00%	40.00%	40.00%
Health and Social Programs
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 210,501	$ 197,225	$ 620,198	$ 511,312
Health and Social Programs | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	42.00%	42.00%	42.00%	39.00%
Security and Other Civilian & Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 86,216	$ 91,550	$ 268,599	$ 270,345
Security and Other Civilian & Commercial | Customer Concentration Risk | Revenue from Contract with Customer
Disaggregation Of Revenue [Line Items]
Revenue from clients, Percent	17.00%	20.00%	18.00%	21.00%